1. Field of the Invention
The present invention relates to radiographic apparatuses for capturing two-dimensional projection image data using a cone-beam X-ray from an X-ray generator and reconstructing three-dimensional image data. Particularly, the present invention relates to a radiographic apparatus that allows a position of a subject relative to an X-ray generator and a sensor to be adjusted based on information obtained from two-dimensional projection-image data.
2. Description of the Related Art
Recently, in order to obtain digital data for a large screen, two-dimensional detectors (also referred to as flat panel detectors (FPDs)) for X-ray imaging are being developed, as disclosed, for example, in Japanese Patent Laid-Open No. 09-288184, corresponding U.S. Pat. Nos. 5,811,790, 5,965,872, and 6,049,074. Particularly, for simple imaging, X-ray imaging apparatuses including two-dimensional detectors with large photoreceptor surfaces having a size of 43 cm×43 cm are coming into practical use. X-ray imaging devices including two-dimensional detectors having large photoreceptor surfaces are referred to as cone-beam computed-tomography (CT) apparatuses (hereinafter referred to as “CBCT apparatuses”).
When imaging is carried out with a CBCT apparatus, an X-ray technologist assumes an approximate position of a target organ of a subject.
Also, helical CT apparatuses are known. In a helical CT apparatus, a subject is irradiated with X-rays. X-rays that transmit through the subject are detected by an X-ray detector, and transparent image data of the subject (referred to as a scanogram or SCOUT image data), a tomogram, or three-dimensional image data is obtained based on the X-ray detection output (the number of photons in the X-rays).
In the helical CT apparatus, in order to determine a scanning region, i.e., a region of a subject to be scanned, scanogram image data or SCOUT image data is obtained. According to techniques disclosed in Japanese Patent Laid-Open No. 07-124151, when the entire lung regions are to be imaged, since the positions of the entire lung regions cannot be identified from the appearance, a range of the entire lung regions is determined based on the scanogram, image data, and a range to be imaged by the CT apparatus is determined while scanning the subject. More specifically, according to the method disclosed, a profile of the pattern of scanogram image data with respect to the direction across the body is analyzed to identify the lung regions.
According to techniques disclosed in Japanese Patent Laid-Open No. 06-304164, laser beams are emitted, a region shielded by a subject is detected by an optical sensor to estimate a chest region of the subject, and the region is helical-scanned.
In a CBCT apparatus, X-rays reach a sensor while spreading in the direction of body axis (the direction of Z axis), so that the resolution of reconstructed image data in the periphery of the sensor with respect to the direction of body axis is lower compared with that in a central region of the sensor. Thus, an imaging system must be positioned so that a region of particular interest in the subject is located in the proximity of the central region of the sensor. However, when the position of a target organ of a subject to be imaged is assumed approximately, it is difficult to locate the target organ suitably in an imaging range of a sensor. Hereinafter, a point where a center of a flux of X-rays emitted from an X-ray generator reaches will be referred to as a radiation center. The radiation center is a point where a center of a flux of X-rays reaches, at which the resolution of image data is usually highest. This is because X-rays perpendicularly cross the sensor surface at the radiation center. Generally, imaging is carried out so that the radiation center matches the center of the sensor.
As for a helical CT apparatus, the cone angle is small, so that a subject is imaged with a plurality of revolutions. That is, a focus of an X-ray generator need not be defined in the subject, and scanogram image data is used to determine a range for imaging a subject by a plurality of revolutions. Thus, according to the method that uses scanogram image data, although it is possible to determine a range for imaging a subject, it is not suggested to set a radiation center, so that it is not possible to determine a radiation center.
Similarly, according to the method that uses laser beams, although it is possible to determine a range for imaging a subject, it is not possible to determine a radiation center.